Thermal conductivity detectors are used to detect certain liquid or gaseous substances (fluids) on the basis of their characteristic thermal conductivity, particularly in gas chromatography. For this purpose, the substances to be detected, after their chromatographic separation, are successively guided past an electrically heated heating filament disposed in a channel. Depending on the thermal conductivity of the substance flowing past, more or less heat is diverted from the heating filament to the channel wall, and the heating filament is correspondingly cooled to a greater or lesser degree. As a result of the cooling of the heating filament, its electrical resistance changes, which is detected. For this purpose, the heating filament is typically disposed in a measuring bridge, which contains additional resistors and an additional heating filament in a further channel through which a reference fluid flows.
To detect very small amounts of substances with great sensitivity and accuracy requires a correspondingly small structure of the thermal conductivity detector. Micromechanical production methods are particularly suitable for this purpose. Due to the small overall size, however, special problems are encountered. For example, the heating filament, which is under tension at an ambient temperature, may relax as a result of its thermal expansion at operating temperatures ranging from 100° C. to 200° C. and above. The relaxed filament may then cause the fluid flowing through the channel to then induce vibrations in the heating filament, which increase the detector noise of the thermal conductivity detector and thereby decrease the detection limit. These vibrations may also cause premature fracturing of the very thin heating filament.